his project attempts to understand the biochemical basis for T ell interactions with other cells in the immune system. uring the past year we have made progress in two areas. irst, studies of antigen-processing, using paraformaldehyde ixed B cell tumors and the antigen pigeon cytochrome c, have hown that this molecule must be cleaved to a fragment onsisting of residues 66-104 in order for it to be stimulatory or T cells. Fragment 60-104 was non-stimulatory. Analysis of he x-ray structure of the molecule suggests that this might be ecause a negatively charged glutamic acid at position 61 ormally interacts with the lysine at position 99, a residue we ave previously shown is critical for T cell activation. When esidues 60 to 65 are eliminated, the lysine at 99 is freed and an then interact with the T cell receptor. n the second area, we have discovered an unusual T cell opulation in low responder animals which can be activated by a 7 amino acid peptide of pigeon cytochrome c in association ith the low responder Ia molecule, but which is blocked from esponding when the intact cytochrome c molecule is present. urprisingly, the blocking appears to be noncompetitive. The elationship of this unusual clone to the low responder henotype of the animal is currently being investigated.